The Emperor tricks Horus to divest himself of the infinite power of Chaos

A trick. A last trick. A last damned trick! A lever to prise open your armour. A feint to make you drop your guard. A magician's encore sleight of hand. The final desperate scheme of an eternal and ruthless schemer.
You make to cast your father's corpse aside, because you understand it is merely part of the trick, but the body is already disintegrating into papery ash and luminous dust. It was just an aspect, another discarded aspect, another empty husk.
He is not dead.
You cry out, in anger and despair. You try to snatch the power back into yourself, but it is pooled around you in a great black slick, sticky and sluggish, slow to respond, slow to obey, reluctant to reinhabit the vessel of your body now that you have scorned it. You draw it back in as quickly as you can. You inhale to fill your lungs and soul with it. You gather it in frantically, for you must be ready to defend yourself.
The worst of it… Your human heart, still raw and exposed, feels relief. A kind of joy. Your father is not dead. Your father is not dead. You didn't kill Him. He lives–
Loken faces you, His sword is in his hand. But it's not Loken. It never was. Loken is still sprawled on the deck to your left where you threw him, gazing on in horror.
Or is it wonder?
You will not die like this. You will not be tricked like this. The power begins to flood back into your veins. The darkness of it. The sweet agony. The reassuring rage. The strength–
Loken steps towards you. The other Loken. The Loken who is not Loken. The sword in his hand is not Rubio's old blade. The sword in His hand is the great war-sword. The face is not Loken's. It is His face. The aspect of Loken collapses into voidmist as your father steps out to meet you in all His bloody majesty.
His wounds are great. Blood is dried black across His face and His ruined arm. But there is a light inside Him, a light behind His eyes, the pure white light of a species that, in its madness, believes in Him beyond all reason, and trusts in Him beyond all logic, a species that imagines Him to be its shield and protector, and has such faith in that act of imagination, it is made real.
He could not fight you alone. He could not beat you alone. But by bluff and ruse and stratagem and sacrifice, He has held your attention until He no longer has to.
To be absent in the body is to be present in the Emperor. That's what the whispers are screaming. A whole species is present here, its will united in one form, not a man, not a father, but a king of all the ages.
He looks like a god. A wounded god, but a god nonetheless. It's not His power, it's where it comes from.
We are one and the same, the whispers say, mankind and Emperor, Emperor and mankind, souls bound together. We are together as one or we are nothing.
'You are no god!' you shout.
Then this will be a fair fight, the whispers answer.
You howl your defiance as He comes at you. He is clearly weak and wounded, but you are weak too. You have gathered up but a fraction of the power you had. You must keep Him at bay for a moment longer, hold Him back while you recover your full strength.
For in this moment, you are just Horus Lupercal.

-Siege of Terra: The End And The Death - Volume three